Rotary drum filter

ABSTRACT

A rotary drum filter comprising a generally annular rotatably driven drum having filtrate compartments which extend generally longitudinally along the drum outer circumference. The filtrate compartments are overlaid by cover elements which have filtrate passages on their outer peripheries inwardly of a foraminous filter medium; and the filtrate passages are communicated with the filtrate compartments by drainage openings in an edge portion of each cover element at intervals along the length of the drum. The drainage openings are constructed to discharge filtrate to their respective communicating filtrate compartments in a direction at least generally towards the discharge ends of the latter whereby the discharging filtrate has a substantial velocity component in such direction.

United States Patent Luthi 1 Aug. 1,1972

[ ROTARY DRUM FILTER [72] Inventor: Oscar Luthi, Nashua, NE.

[73] Assignee: Improved Machinery Inc., Nashua,

[22] Filed: Sept. 10, 1970 [21] Appl. No.: 71,073

[52] US. Cl. ..210/404 [51 Int. Cl. ..Btlld 33/06 {58] Field of Search..210/479, 492, 399-407 [56] References Cited UNITED STATES PATENTS1,470,463 10/1923 Manning ..210/399 X 1,949,998 3/ 1934 French....-..210/402 X 3,327,863 6/1967 Turner, Jr. ..210/404 X 3,517,818 6/1970Luthi ..210/404 X 3,306,457 2/1967 Purnam ..210/404 X PrimaryExaminerReuben Friedman Assistant Examiner-T. A. Granger Attorney-CarlR. Horten, David W. Tibbott and Robert R. Paquin [57] ABSTRACT A rotarydrum filter comprising a generally annular rotatably driven drum havingfiltrate compartments which extend generally longitudinally along thedrum outer circumference. The filtrate compartments are overlaid bycover elements which have filtrate passages on their outer peripheriesinwardly of a foraminous filter medium; and the filtrate passages arecommunicated with the filtrate compartments by drainage openings in anedge portion of each cover element at intervals along the length of thedrum. The drainage openings are constructed to discharge filtrate totheir respective communicating filtrate compartments in a direction atleast generally towards the discharge ends of the latter whereby thedischarging filtrate has a substantial velocity component in suchdirection.

16 Claims, 5 Drawing Figures P'A'TE'N'TEDAus 1 I972 4o 2 FIG. 44 24 IO22 INVENTOR OSCAR LUTH/ ATTOR EY ROTARY DRUM FILTER The presentinvention relates to filtering apparatus and more particularly to rotarydrum filters of the type for filtering a slurry such as, for example,pulp stock for the manufacture of paper.

Conventionally, rotary drum vacuum filters have been constructed toinclude a generally annular rotatably driven drum having filtratecompartments extending generally longitudinally along its outercircumference, and cover elements overlaying the filtrate compartmentsspaced one from another circumferentially of the drum to permit filtratedrainage to the compartments between the cover elements. Also, theseconventional filters have been provided with filtrate passages on theouter circumferences of the cover elements inwardly of a foraminousfilter medium which may be supported by one longitudinal edge of each ofthe cover elements. Such a filter is, for example, described in U.S.Pat. No. 3,306,460, issued Feb. 28, 1967, to Oscar Luthi and assigned tothe assignee of the present invention.

During the operation of these conventional filters, filtrate flowinginwardly through the filter medium into the filtrate passages isdirected by the cover elements in the circumferential direction of thedrum to the filtrate compartments which in turn direct the filtrateIongitudinally of the drum. It has been discovered that during thisoperation, and particularly when such a filter is operating at arelatively high flow rate, a pressure dif-.

ferential occurs between the discharge ends of the filtrate compartmentsand the other or remote ends of such compartments. More specifically,the subatmospheric pressure or vacuum applied to the remote ends of thecompartments may be substantially lower than that applied to thecompartment discharge ends, thereby resulting in a correspondingly lowerrate of filtrate flow through the filter medium adjacent such remoteends and less fiber deposit on the filter medium at such location. Thisnonuniformity in fiber build-up on the filter medium is detrimental tofilter efficiency and, particularly in extreme cases, can permitexcessive introduction of air which undesirably lessens the effectiveoperating subatmospheric pressure or vacuum of the filter. It isbelieved that this pressure differential is primarily due to fluidfriction in the compartments and the energy necessary to accelerate thefiltrate, entering the compartments circumferentially of the drum withno velocity longitudinally of the compartments, towards the dischargeends of the compartments.

An object of the present invention is to provide a new and improvedrotary drum filter which is particularly constructed and arranged tosubstantially minimize the aforesaid pressure differentials in thefiltrate compartments.

Another object of the invention is to provide a new and improved rotarydrum filter which is particularly constructed and arranged whereby' thefiltrate is discharged to the filtrate compartments'with a substantialvelocity component in the longitudinal direction of the compartments.

Another object is to provide a new and improved rotary drum filter whichis particularly constructed and arranged to provide these beneficialresults in a relatively simple and economical manner.

These objects, and other objects and advantages of the invention whichwill be apparent from the following description taken in connection withthe accompanying drawings, may be attained by the construction of a drumstructure for a rotary drum filter to, in general, comprise a generallyannular drum, a, plurality of rib elements extending generallylongitudinally along the circumference of said drum andcircumferentially spaced around the drum to define a plurality offiltrate compartments extending generally longitudinally along the drumcircumference, a plurality of cover elements mounted generallylongitudinally and circumferentially around the drum laterally spacedfrom the drum circumference by the filtrate compartments, a foraminousfilter medium overlying the cover elements and spaced therefrom toprovide filtrate passages intermediate the cover elements and the filtermedium, thecover elements being provided with filtrate drainage openingsconnecting the filtrate passages with the filtrate compartments forsupplying filtrate from the former to the latter, and louver meansadjacent the drainage openings for causing filtrate draining through thedrainage openings to be discharged to the filtrate compartments in adirection at least generally longitudinally of the filtratecompartments.

Referring to the drawings:

FIG. 1 is an elevational sectional view of a rotary drum filterconstructed in accordance with the present invention;

FIG. 2 is an elevational sectional view of the filter taken on line 2-2of FIG. 1, looking in the direction of the arrows;

FIG. 3 is an enlarged, fragmentary elevational sectional view taken online 3-3 of FIG. 1, looking in the direction of the arrows;

FIG. 4 is an enlarged, fragmentary perspective view, partially brokenaway and 'in-section, illustrating the cover element and filtratecompartments of the filter shown in FIGS. 1 through 3, looking from thedischarge ends of the compartments; and

FIG. 5 is an enlargedpe'rspective view fragmentarily illustratinganother filter constructed in accordance with the invention.

Referring more particularly to the drawings wherein similar referencecharacters designate corresponding parts throughout the several views,the illustrated rotary drum filter comprises a generally annularrotatably driven drum or drum structure designated generally as 10 whichis partially submerged in a tank or vat 12 containing a slurry includingpulp fibers. The drum 10 is formed to include a pair of end closureplates 14 each connected to a trunnion 16, which serves to rotatablysupport the drum 10 on a bearing pedestal 18. One of the trunnions 16 ishollow and has the upper end of a barometric leg or vacuum pipe 20extending thereinto, such barometric leg 20 during the operation of thedrum filter operating in the conventional manner to drain filtrate andcreate and apply a subatmospheric pressure or vacuum.

As illustrated, the drum 10 is constructed to include two identicalcoaxial annular drum halves 22 spaced one from another by an annularspace or channel 24 adjacent the longitudinal mid-point of the drum 10.The drum halves 22 are connected to one another by suitable conventionalbracing elements such as the tie Y 3 elements 26, to be conjointlyrotatable and preferably both along their outer circumferences slopedownwardly towards the space or channel 24 to facilitate. filtratedrainage thereto. However, the

present invention is not limited to filters in which the drum is dividedinto halves or other sections but rather, as will become readilyapparent from the following description, is equally applicable tofilters wherein the drum'is undivided in which event'the entire drumcould, for example, be constructed similarly to a single one of theillustrated drum halves 22.

A plurality of imperforate laterally upstanding rib elements 28 arerigidly mounted on the outer circumference of each drum half 22 toextend generally longitudinally along such outer circumferencesubstantially throughout the length of the drum half 22. The imperforaterib elements 28 of each drum half '22 are circumferentially spacedaround the drum half 22 to cause circumferentially adjacent elements 28to form opposing side walls of filtrate compartments 30 which extendgenerally longitudinally along the'outer circumference of the drum half22 substantially throughout its extent; and intermediate each twocircumferentially adjacent imperforate rib elements 28 there areprovided other rib elements 32, 34 which have openings 36 37,respectively, therethrough communicating the portions of the filtratecompartments 30 on opposite sides thereof-The filtrate compartments 30are each closedat one end by'an end-closure plate 14, closed along theirinner periphery by'the outer circumference of the respective drum half22, and have juxtaposed discharge ends 30a open to communication withthe annularspace 24 between the drum halves 22.

As illustrated, a stationary or non-rotatable valving assemblydesignated generally as 38 is located in the space 24. The valvingassembly 38 is constructed such that, although permitting theapplication of subatmospheric pressure or vacuum to the filtratecompartments 30 through the compartment ends 30a during the majorportion of the drum rotation, it prevents such applicationofsubatmospheric pressure from about the l oclockposition (as viewed'inFIG.2) through about the 4 o'clock positiontas viewed in FIG. 2). Thevalving assembly 38 is shown as being held against rotation by meansexternallyof the drum 10, but may be alteropposite sides of a fluidpassage 46 communicating with the open inner periphery of the valvingassembly 38 throughout this 270? arcuate length. The fluid passage 46,in turn, is connected through the channel 48 in a hollow trunnion l6tothe barometric leg 20 to thereby communicate the barometric leg 20with the aforesaid 270 arcuate length ofthe valving assembly 38.

The valving assembly 38 from the aforesaid about 1 oclock positionthrough the aforesaid about 4 o'clock position is closed along its innerperiphery by an arcuate wall which prevents-communication of thefiltrate compartments 30 with the barometric leg 20. Also, this latterlength of the valving assembly 38 is closed at its opposite ends bytransversely extending end closure walls (not shown) and along itsopposite sideshas side walls 50, one of which is shown in FIG. 2.-Theside walls 50 are each provided with an elongated opening 52therethrough; and the outer wall 40 is intheconventional manner providedwith openings (not shown) permitting air fromthe initially submergingfiltrate compartments 30 to escape to the atmosphere through theelongated openings 52 and such openings in the wall 40. Further detailsof the construction of the illustrated valving assembly 38 may beobtained from the aforesaid US. Pat. No. 3,306,460.

A plurality of cover elements or plates 54 are rigidly mounted on theradially outer endsof' the rib elements 28, 32, 34 to extend generallylongitudinally and circumfer'entially around the outer circumference ofthe drum l0 radially spaced outwardly from such outer cirnativelyheld-stationary in any other conventional manner such as, for example,by means internally of the drum 10. Furthermore, alternatively to theillustrated valving assembly 38 the filter could, if desired, beprovided with a valve which is located at one end of the drum andconnected to the compartment ends 30a by a plurality of separate pipes.This latter form of valve is,

for example, described in IJ.S; Pat. No. 3,363,774, issued .Ian. 16,1968, to Oscar Luthi and assigned to th assignee of the presentinvention.

The illustrated valving assembly 38 comprises an arcuate outer wall 40which surrounds the drum 10 and throughout the approximately 270 arebetween about the 4 oclock position of FIG. 2 and about the 1 o'clockposition of FIG. 2-has integral side walls 42 provided with openings .44communicating with the open ends 30a of the compartments 30 of each drumhalf 22. The

inner periphery of the valving assembly 38 is open throughout suchapproximately 270 arc; and the drum 10 is provided with spaced discs 43,45 bounding the cumference by the filtrate compartments 30 of whichtending generally in the longitudinal direction of "the drum 10, theedge portion 54b of each coverelement 54, as shown in FIGS. 3 and'4,being bent outwardly from the drum 10 at, for example, a 90 angle.

Channel or division strips 56 may be provided on the outer circumferenceof each cover element 54, such channel strips56 extending generallycircumferentially of the drum l0 and being spaced by therebetweenfiltrate passages or channels 60 which'similarly extend in the generallycircumferential direction of the drum 10. The channel strips 56 may, ifdesired, each have an end affixed in a groove (not shown) in theoutwardly "extending edge portion 54b of a cover element 54. A

foraminous sheet filter medium .58 is wrapped aroundintervals along its"length providedwith aligned drainage openings 62 communicating theunderlying filtrate passages with an overlaid filtrate compartment 30for draining filtrate from such filtrate passages 60 to such filtratecompartment 30. The drainage openings 62 are each constructed tolongitudinally extend generally transversely to the respective connectedfiltrate compartment 30 and discharge filtrate to such compartment 30 ina direction at least' generally towards the discharge end 30a of thelatter. In this manner, the discharging filtrate is thereby providedwith a substantial velocity component in the longitudinal direction ofthe filtrate compartment 30 towards its discharge end 30a.

More particularly, as best illustrated in FIG. 4, the edge portion 54aof each cover element 54 is at intervals along its length provided withportions 64 laterally offset or recessed inwardly towards the therebelowfiltrate compartment 30; and the drainage openings 62 are each locatedat the end of arespective recessed portion 64 most adjacent to thedischarge end 30a of the communicating filtrate compartment 30.Resultantly, all of the drainage openings 62 in each cover element 54(excepting the first one in each cover element 54) are formed generallyof slot-like louvered configuration. The recessed portions 64 slopedownwardly towards their respective drainage openings 62 and areadjustable by bending to vary the flow volumes of their respectivedrainage openings 62, such adjustment of one of the recessed portions 64being depicted by broken lines in FIG. 3.

Also, as shown-in FIG. 5, the recessed portions 64 may, if desired, eachgradually widen as they extend towards their respective drainageopenings 62.

In operation, the drum is rotatably driven in the clock-wise direction,as viewed in FIG. 2, to successively drive the filtrate compartments 30under the level of the slurry in the vat 12 for filter cake accretionand then above such level for drying of the cake and removal of theaccumulated cake from the filter medium 58 by doctor means (not shown)generally at the lo.- cation of the roller 66 during the initialsubmerging of the filtrate compartments 30 into the slurry, filter cakeaccretion is effected without the assistance of the subatmosphericpressure but air in the initially submerging filtrate compartments 30 ispermitted to escape to the atmosphere. Below the lowermost ends of thewalls 50 subatmospheric pressure is applied to the filtrate compartments30 through their ends 300 and additional filter cake is accumulated onthe filter medium 58 due to this application of subatmospheric pressure.The subatmospheric pressure continues to be applied to the compartments30 through their open ends 30a as the compartments 30 rotate above thelevel of the slurry and the accumulated filter cake is thereby dried.The subatmospheric pressure is cut off from the compartments 30 as theyrotate to the upper end of the wall 50 where the dried accumulatedfilter cake is removed from the filter medium preparatory to thecommencement of another filter cycle.

Throughout the drainage of filtrate through the filtrate compartments 30due to the application of subatmospheric pressure, the filtrate flow isas schematically depicted by the arrows in FIG. 4. More particularly,the filtrate flows inwardly through the filter medium 58 onto the outercircumference of the cover elements 54 and thence flowscircumferentially across the cover elements 54 to the recessed portions64 at the edge portions 54a. T he. recessed portions 64 direct thisfiltrate to the drainage openings 62 which discharge the filtrate intothe filtrate compartments 30 in a direction generally towards thedischarge ends 300. Thus, the filtrate is discharged by the drainageopenings 62 with a substantial velocity component in the longitudinaldirection of the filtrate compartments 30 towards the discharge ends 30aand the beforedescribed objects are resultantly attained.

It will be understood that, although only one embodiment of theinvention has been illustrated and specifically hereinbefore' described,the invention is not limited merely to this single embodiment but ratherincludes other embodiments and variations within the scope of thefollowing claims.

Having thus described my invention, I claim:

1. A drum structure for a rotary drum filter, comprising a generallyannular rotatable drum, a plurality of rib elements extending generallylongitudinally along the circumference of said drum andcircumferentially spaced around said drum to define a plurality offiltrate compartments extending generally longitudinally along the drumcircumference, a plurality of cover elements mounted generallylongitudinally and circumferentially around said drum radially spacedfrom the drum circumference by said filtrate compartments, a foraminousfilter medium overlying said cover elements and spaced from therefrom toprovide filtrate passages intermediate said cover elements and saidfilter medium, said cover elements being provided with filtrate drainageopenings connecting said filtrate passages with said filtratecompartments for draining filtrate from said filtrate passages to saidfiltrate compartments, and louver means adjacent said drainage openingsfor causing filtrate draining through said drainage openings to bedischarged to said filtrate compartments in a direction at leastgenerally longitudinally of said filtrate compartments.

2. A drum structure according to claim 1, wherein said louver means areunitary with said cover elements and slope inwardly from said coverelements into said filtrate compartments.

3. A drum structure according to claim'2, wherein said drainage openingsare of generally slot-like configuration.

4. A drum structure according to claim 2, wherein the drainage openingsof each cover element are at least generally aligned in the longitudinaldirection of said drum.

5. A drum structure for a rotary drum filter, comprising a generallyannular rotatable drum, a plurality of generally upstanding rib elementsextending generally longitudinally along the outer circumference of saiddrum and circumferentially spaced around said drum one from another todefine a plurality of filtrate compartments extending generallylongitudinally along the outer circumference of said drum, a pluralityof cover elements. mounted generally longitudinally andcircumferentially around said drum radially spaced outwardly from theouter circumference of said drum by said filtrate compartments, aforaminous filter medium overlying said cover elements and spacedoutwardly therefrom to provide filtrate passages intermediate saidfilter medium and the outer circumferences of said cover elements, saidcover elements being provided at intervals along their lengths withdrainage openings passages to said filtrate compartments and saidfiltrate compartments having discharge ends at which the filtrate isdischarged therefrom, and louvers adjacent said drainage openings forcausing filtrate draining through 1 said drainage openings to saidfiltrate compartments to be discharged to said filtrate compartments ina direction at least generally towards said discharge ends of saidfiltrate compartments.

- 6. A drum structure according to claim 5, wherein said louvers areintegral with said cover elements.

7. A drum structure according to claim 5, wherein said louverscompriseportions of said cover elements recessed into said filtrate compartmentsand said drainage openings are located adjacent the end of theirrespective recessed portions most adjacent to the discharge end of thecommunicating filtrate compartment. v v

8.' A drum structure according to claim 5, wherein said louvers compriseportions of each cover element laterally offset from the main portion ofthe cover element, and said drainage openings are each located adjacentthe ends of their ofiset portions most adjacent to the discharge end ofthe communicating filtrate compartment. v

9. A drum structure according to claim 5, wherein the drainage openingsof each cover element are along an edge portion of the cover tendinggenerally longitudinally along the outer circumference of said. drumsubstantially throughout its extent, a plurality of cover elementsmounted on said rib elementsoverlying the outer circumference of saiddrum and radially spaced outwardly from the drum outer circumference bysaid filtrate compartments, a

foraminous sheet filter medium overlying said cover elements andradiallyoutwardly spaced therefrom by drainage passages between said coverelements and said filter medium, each said cover element having an edgeportion generally in the longitudinal direction of said drum whichoverlies'a portion of a filtrate compartment, saidedge portion of eachsaid cover element at intervals along its length having transverselyextending drainage openings communicating an overlying filtrate passagewith an overlaid filtrate compartment for draining filtrate from suchfiltrate passage to such filtrate compartments, said filtratecompartments having discharge ends at which the filtrate is dischargedtherefrom, and said cover elements including louvers adjacent saiddrainage openings for causing filtrate draining through said drainageopenings to' be discharged thereby in a direction towards said dischargeends of said filtrate compartments.

11. A drum structure according toclaim 10, wherein each said coverelement at intervals along the length of said edge portion is laterallyofi'set towards a filtrate compartment to provide said louvers, and saiddrainage Siii i fififs fiiili ififi hi fo ili$iii$fifli thecommunicating filtrate compartment.

12. A drum structure according to claim 11, wherein such offset portionsare adjustable to vary the flow volumes of said drainage openings.

13. A drum structure according to claim 11, wherein said cover elementscooperate to prevent substantial filtrate flow from said filtratepassages to said filtrate compartments other than through said drainageopenings.

14. A drum structure according to claim 10, wherein said drainageopenings'are provided in the corresponding edge portion of each coverelement.

15. A drum structure according to claim 1 1, wherein said ends of theoffset portions are wider than the other ends thereof.

16. A drum structure according to claim 1 1, wherein said offsetportions of said cover elements each gradually widen as they extendtowards their said ends.

1. A drum structure for a rotary drum filter, comprising a generallyannular rotatable drum, a plurality of rib elements extending generallylongitudinally along the circumference of said drum andcircumferentially spaced around said drum to define a plurality offiltrate compartments extending generally longitudinally along the drumcircumference, a plurality of cover elements mounted generallylongitudinally and circumferentially around said drum radially spacedfrom the drum circumference by said filtrate compartments, a foraminousfilter medium overlying said cover elements and spaced from therefrom toprovide filtrate passages intermediate said cover elements and saidfilter medium, said cover elements being provided with filtrate drainageopenings connecting said filtrate passages with said filtratecompartments for draining filtrate from said filtrate passages to saidfiltrate compartments, and louver means adjacent said drainage openingsfor causing filtrate draining through said drainage oPenings to bedischarged to said filtrate compartments in a direction at leastgenerally longitudinally of said filtrate compartments.
 2. A drumstructure according to claim 1, wherein said louver means are unitarywith said cover elements and slope inwardly from said cover elementsinto said filtrate compartments.
 3. A drum structure according to claim2, wherein said drainage openings are of generally slot-likeconfiguration.
 4. A drum structure according to claim 2, wherein thedrainage openings of each cover element are at least generally alignedin the longitudinal direction of said drum.
 5. A drum structure for arotary drum filter, comprising a generally annular rotatable drum, aplurality of generally upstanding rib elements extending generallylongitudinally along the outer circumference of said drum andcircumferentially spaced around said drum one from another to define aplurality of filtrate compartments extending generally longitudinallyalong the outer circumference of said drum, a plurality of coverelements mounted generally longitudinally and circumferentially aroundsaid drum radially spaced outwardly from the outer circumference of saiddrum by said filtrate compartments, a foraminous filter medium overlyingsaid cover elements and spaced outwardly therefrom to provide filtratepassages intermediate said filter medium and the outer circumferences ofsaid cover elements, said cover elements being provided at intervalsalong their lengths with drainage openings communicating said filtratepassages with said filtrate compartments for draining filtrate from saidfiltrate passages to said filtrate compartments and said filtratecompartments having discharge ends at which the filtrate is dischargedtherefrom, and louvers adjacent said drainage openings for causingfiltrate draining through said drainage openings to said filtratecompartments to be discharged to said filtrate compartments in adirection at least generally towards said discharge ends of saidfiltrate compartments.
 6. A drum structure according to claim 5, whereinsaid louvers are integral with said cover elements.
 7. A drum structureaccording to claim 5, wherein said louvers comprise portions of saidcover elements recessed into said filtrate compartments and saiddrainage openings are located adjacent the end of their respectiverecessed portions most adjacent to the discharge end of thecommunicating filtrate compartment.
 8. A drum structure according toclaim 5, wherein said louvers comprise portions of each cover elementlaterally offset from the main portion of the cover element, and saiddrainage openings are each located adjacent the ends of their offsetportions most adjacent to the discharge end of the communicatingfiltrate compartment.
 9. A drum structure according to claim 5, whereinthe drainage openings of each cover element are generally aligned alongan edge portion of the cover element.
 10. A drum structure for a rotarydrum filter, comprising a generally annular rotatable drum, a pluralityof generally laterally upstanding rib elements extending generallylongitudinally along the outer circumference of said drum andcircumferentially spaced around said drum to define a plurality offiltrate compartments extending generally longitudinally along the outercircumference of said drum substantially throughout its extent, aplurality of cover elements mounted on said rib elements overlying theouter circumference of said drum and radially spaced outwardly from thedrum outer circumference by said filtrate compartments, a foraminoussheet filter medium overlying said cover elements and radially outwardlyspaced therefrom by drainage passages between said cover elements andsaid filter medium, each said cover element having an edge portiongenerally in the longitudinal direction of said drum which overlies aportion of a filtrate compartment, said edge portion of each said coverelement at intervals along its length having transversely extendingdrainage openings Communicating an overlying filtrate passage with anoverlaid filtrate compartment for draining filtrate from such filtratepassage to such filtrate compartments, said filtrate compartments havingdischarge ends at which the filtrate is discharged therefrom, and saidcover elements including louvers adjacent said drainage openings forcausing filtrate draining through said drainage openings to bedischarged thereby in a direction towards said discharge ends of saidfiltrate compartments.
 11. A drum structure according to claim 10,wherein each said cover element at intervals along the length of saidedge portion is laterally offset towards a filtrate compartment toprovide said louvers, and said drainage openings are each located at theend of their respective offset portions most adjacent to the dischargeend of the communicating filtrate compartment.
 12. A drum structureaccording to claim 11, wherein such offset portions are adjustable tovary the flow volumes of said drainage openings.
 13. A drum structureaccording to claim 11, wherein said cover elements cooperate to preventsubstantial filtrate flow from said filtrate passages to said filtratecompartments other than through said drainage openings.
 14. A drumstructure according to claim 10, wherein said drainage openings areprovided in the corresponding edge portion of each cover element.
 15. Adrum structure according to claim 11, wherein said ends of the offsetportions are wider than the other ends thereof.
 16. A drum structureaccording to claim 11, wherein said offset portions of said coverelements each gradually widen as they extend towards their said ends.